On February 1, 2017, the National Institute for Occupational Safety and Health (NIOSH) posted a blog item entitled “The Art and Science of OELs for Nanomaterials.” According to the item, NIOSH searched for proposals for occupational exposure limits (OEL) for new nanomaterials as part of the development of a World Health Organization (WHO) guideline for working safely with nanomaterials. The blog item cites the article “Occupational exposure limits for manufactured nanomaterials, a systematic review,” which Nanotoxicology published last month. The blog item notes that the time to develop test data and the rapid increase in the number of nanomaterials can make the traditional method of quantitative risk assessment for OEL derivation difficult. As a result, scientists have proposed other methods:
- The properties of a new nanomaterial can be compared to an existing chemical substance with similar properties, as has been done with nanofibers and asbestos;
- A safety factor can be applied to the nanoscale variant of existing chemicals with known toxic effects, such as cancer, as proposed by the British Standard Institute;
- Using the knowledge available from air pollution research; and
- Using quantitative structure-activity relationship (QSAR) modeling to predict the toxicity, although the blog item states that this approach appears not to have been used yet for nanomaterial OEL derivation.
The blog item notes that it is unclear what measurement is the best for OELs for engineered nanomaterials — mass concentration (milligrams per cubic meter), number concentration (number of particles in a cubic meter), or surface concentration (the total surface of the particles in a cubic meter). When searching for proposed OELs, only about half were published in peer-reviewed journals, while the rest were “scattered across project reports published on the Internet only, conference abstracts or government reports.” The OELs found varied in many cases, ranging from a factor of 50 for OELs for carbon nanotubes to even more for OELs for metallic nanomaterials. The blog item concludes that “[t]he challenge is to find consensus on how to derive the OELs for nanomaterials and next to underpin the proposed values with more empirical research.” According to NIOSH, at this time, “the OELs for nanomaterials are bench mark levels to which the exposure levels measured in a specific workplace can be compared. Compared to the concentrations that have been measured in workplaces, these OEL benchmark levels are often still substantially lower and should be an incentive to reduce exposure.”